A hard disk drive (hereinafter referred to as an HDD) is an example of a magnetic recording and reproducing apparatus including a magnetic recording medium. In the HDD, a magnetic sensor in a magnetic reproducing head reads out magnetic information recorded on a magnetic disk corresponding to the magnetic recording medium. As the magnetic sensor of the magnetic reproducing head, a magnetoresistance effect sensor such as a GMR sensor or TMR sensor is conventionally used.
The magnetic recording density of the HDD is increasing year by year. The maximum surface recording density of HDDs commercially available in 2011 is about 700 Gbit/in2. According to the HDD technical road maps, the surface recording density will reach 1 Tbit/in2 in about 2013, and 2 Tbit/in2 in about 2015.
Increasing the surface recording density is equal to decreasing the size of a medium bit in the magnetic disk. The size of the magnetic sensor must be decreased so as to match the medium bit size. When the medium bit size and magnetic sensor size further decrease in the future, noise in reproduced signals increases, and this makes the conventional magnetoresistance effect sensors unable to secure a practical signal-to-noise ratio. The main causes that decrease the signal-to-noise ratio are, e.g., thermal mag-noise and inter-bit interference noise. The thermal mag-noise is noise caused by the thermal fluctuation of magnetization of a magnetic layer in the magnetic sensor. The inter-bit interference noise is noise caused when a magnetic field generated by a medium bit other than a target bit, which is a medium bit to be read out, acts on the magnetic sensor.
As a magnetic sensor capable of suppressing the thermal mag-noise, a spin-torque oscillator magnetic sensor using the spin transfer effect has been proposed. The spin-torque oscillator (STO) has as its basic structure a multilayered film in which a magnetization free layer, a spacer layer, and a ferromagnetic layer (e.g., a magnetization pinned layer) are sequentially stacked. When an electric current is supplied to the spin-torque oscillator, the magnetization pinned layer spin-polarizes the electric current, and the spin-polarized electric current makes magnetization in the magnetization free layer oscillate stably. A magnetic reproducing head using the spin-torque oscillator as a magnetic sensor (this head is also called a reproducing head with a spin-torque oscillator, and will be hereinafter referred to as an STO reproducing head) can read out information from the magnetic recording medium by using a phenomenon in which the amplitude and frequency of magnetization oscillation depend on an external magnetic field acting on the spin-torque oscillator. When the oscillatory energy of magnetization is much higher than the thermal energy, the thermal fluctuation of magnetization is relatively suppressed. Accordingly, it is possible to avoid the problem of the thermal mag-noise by using the spin-torque oscillator as the magnetic sensor.
On the other hand, the inter-bit interference noise may be suppressed by forming magnetic shields so as to absorb magnetic fields from medium bits other than a target bit. The magnetic shields are provided such that the magnetic sensor is positioned therebetween. The decrease of the medium bit size may be dealt with by decreasing the gap between the magnetic shields. Since the magnetic sensor is arranged between the magnetic shields, however, the gap between the magnetic shields cannot be set smaller than the thickness of the multilayered film of the magnetic sensor. This limits the suppression of the inter-bit interference noise by decreasing the gap between the magnetic shields.
As described above, the problem of the thermal mag-noise may be avoided by using the spin-torque oscillator as the magnetic sensor, but the reduction in inter-bit interference noise is limited. Therefore, the STO reproducing head is required to be able to reduce the inter-bit interference noise in a reproduced signal.